I don't go for the dual desire theory. It's not as if there would be any point trying to represent two Halo weapons rather than only one. It would almost be like having a sequence where a character gets into a car, and then a few moments later they're driving a different car. You wouldn't get that just because two animators each wanted to draw their own type of car. With the weapon discrepancy it's not going to be so noticeable to most viewers, but still, it would be a distraction for no good reason (not that there was much humour to be distracted from; this episode seemed really weak to me). Perhaps when the animator tried drawing the guy with the gun, they thought it didn't look right in some way; e.g. maybe the trigger area was awkward for showing the fingers or something. Or maybe someone decided it just didn't look enough like a typical gun.